Optical head apparatus

Abstract: The present invention provides an optical head apparatus for a magneto-optical recording medium. The apparatus comprises a light beam source (61) for emitting a light beam, an objective lens (62) which focuses the light beam emitted from said light beam source (61) to the recording surface of a magneto-optical recording medium (110), a separating optical system (63-67) which separates the beam into a light beam emitted from said light beam source (61) and the reflected light beam reflected from the recording surface of the magneto-optical recording medium (110) via said objective lens (62), and a photodetector (68) for receiving said reflected light beam reflected from the recording surface, being separated by said separating optical system (63-67). The separating optical system (63-67) comprises a Wollaston prism (66) made of lithium niobate placed in the upstream side of said photodetector (68) and in the optical path of said reflective light beam reflected from the recording surface (110). Furthermore, the Wollaston prism (66) is formed by adhering a pair of lithium niobate optical prisms (66a, 66b) to each other, in such a manner that the crystal optical axis of the prism on the side for light beam incidence of said reflected light beam from the recording surface is tilted at an angle of 45° with respect to the polarization plane of said reflected light beam from the recording surface (110), and that the crystal optical axis of the prism on the side for emitting said reflected light beam from the recording surface (110) is tilted at an angle of 90° with respect to the polarization plane of said reflected light beam from the recording surface (110).

Description: BACKGROUND 1. Field of the Invention The present invention relates to an optical head. More particularly, the present invention relates to an optical head for recording signals onto an optical recording medium such as a magneto-optical recording medium, or for reading signals therefrom. 2. Background of the Invention In the case of an optical head for reading out information signals from a rewritable magneto-optical disc they can be fabricated, for example, into a structure illustrated in FIG. 1. Referring to FIG. 1, an optical head 1 includes a number of optical elements described below. A light beam, emitted from a semiconductor laser element 2 as a light beam source, is introduced into an objective lens 3, so that the light beam can be converged on the signal recording surface of the magneto-optical disc MO. The divergent light beam which is emitted from the semiconductor laser element 1 is converted into parallel light beams by a collimator lens 4. A beam splitter 5, which includes an optical prism (hereinafter referred to simply as a "prism") 51 adhered to an optical element 52 having its edges arranged in parallel with each other, separates the light beam into emitted by the semiconductor laser element 1 from a reflected light beam, i.e., a light beam reflected by the signal recording surface of the magneto-optical disc MO. In further detail, the beam splitter 5 has a multilayered dielectric layer in the boundary plane between the prism 51 and the optical element 52, that is, between the facet (edge plane) 5a of the optical element and the prism 51. This multilayered dielectric layer transmits the P-polarization component, but reflects the S-polarization component. A total reflection film which completely reflects the reflected light beam from the recording surface of the magneto-optical disc MO is provided on the other facet (edge plane) 5b of the optical element 52. The dielectric layer and the total reflection film are provided by means of vapor deposition and the like. A Wollaston prism 6 is provided to emit a plurality of beams based on the aforementioned reflected light beam from the signal recording surface. A Wollaston prism as disclosed, for example, in EP-A-0 270 874 can be used in the present invention. The plurality of beams discharged from the Wollaston prism 6 are focused on the photodetector 9 by means of an imaging lens 7. A multi-purpose lens 8 is placed between the imaging lens 7 and the photodetector 9 to generate astigmatism for the detection of focusing error. Furthermore, since the multi-purpose lens 9 includes a concave plane on the output side, the optical path of the reflected light beam between the output end plane of the beam splitter 5 and the photodetector 9 can be shortened. The photodetector 9 has a plurality of photoreceptors for generating each of the following error signals; a focusing error signal and a tracking error signal. Then, signals according to the read-out signal from the information signal recorded on the signal recording surface of a magneto-optical disc MO are generated based on the detection signal provided by the photodetector 9. According to the optical head 1 according to the constitution above, the divergent P-polarization light beam emitted from a semiconductor laser element 1 is converted into parallel light beams by a collimator lens 4. The output light beam from the collimator lens 4 is then focused to one point on the signal recording surface of the magneto-optical disc MO by the objective lens 3 that is provided after the beam splitter 5. The light beams inclusive of the S-polarization component and the reflected light beam, which had been reflected by the signal recording surface of the magneto-optical disc MO, are again introduced to the beam splitter 51 via the objective lens 3. Then, the light beams are reflected by the boundary plane of the beam splitter and their optical path is deflected at an angle to 90°. They are further reflected completely and further deflected at the angle to 90° at the face 5b of the optical element 52. The Wollaston prism 6 emits a plurality of light beams according to the light beam irradiated by the beam splitter 5, so that the plurality of light beams which are provided to the photodetector 9 via a multi-purpose lens 8 might be focused on each of the photoreceptors of the photodetector 9. It should be noted that the light beam passed through the multi-purpose lens 8 has stigmatic aberration. Then, signals such as the signals read out from the magneto-optical disc MO, the focusing error signal, and the tracking error signal, are generated based on the detected signals from each of the photoreceptor planes of the photodetector 9. The signals recorded on the magneto-optical disc MO is read out based on the optical rotation of the beam according to Kerr effect. The optical head 1 of Fig. 1 has an artificial quartz (SiO 2 ) as the Wollaston prism. The difference of indices of refraction Δn, as expressed by the equation below, of this Wollaston prism upon using a light beam source with a wavelength λ of 780 nm is about 0.0089: Δn ≡ n e - n o where, n e is the index of refraction of an extraordinary light beam, and n o is the same of an ordinary light beam. In the case of a Wollaston prism using an artificial quartz, the separating angle between the extraordinary light beam and the ordinary light beam is about one degree when the prism is adhered at an angle of 45° with respect to the crystallographic orientation. Thus, if a compact optical head were to be implemented by the constitution above, the Wollaston prism should be provided at a thickness of at least 4 mm. Accordingly, a compact head cannot be accomplished as long as it is based on this constitution. SUMMARY OF THE INVENTION It is, therefore, and object of the present invention to provide an optical head apparatus which resolves the aforementioned problem. According to the present invention, there is provided an optical head apparatus for a magneto-optical recording medium, which includes alight beam source, an objective lens, a separating optical system, and a photodetector, wherein, the objective lens is provided for irradiating a light beam radiated from a light beam source as a focused light beam on the surface of the signal recording surface of a magneto-optical recording medium; the separating optical system separates the light beam emitted by the light beam source from a reflected light beam reflected by the signal recording surface of the optical recording medium and passed through the objective lens; and the photodetector receives the reflected light beam from the signal recording surface and separated by the separating optical system. The separating optical system further includes a lithium niobate Wollaston prism placed in the photodetector side of the optical path of the reflected light beam reflected by the signal recording surface. The present invention provides a simple-structured compact optical head. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more readily understood with reference to the accompanying drawings, wherein: FIG. 1 shows a schematically drawn structure of a prior art optical head; FIG. 2 shows a schematically drawn structure of an optical head according to the present invention; and FIGS. 3 (A) to 3 (C) are each explanatory drawings to show the structure of a Wollaston prism according to the present invention, wherein, FIG. 3 (A) is provided as an explanatory means showing the generation of a plurality of light beams upon incidence of a light beam to a Wollaston prism, FIG. 3 (B) is provided as another explanatory means showing crystal optical axes on the incident beam side of a Wollaston prism, and FIG. 3 (C) is provided as a still another explanatory means showing crystal optical axes on the output side of a Wollaston prism. DESCRIPTION OF THE INVENTION The present invention is illustrated in greater detail referring to the drawings. Referring to FIG. 2, an optical head according to the present invention is described below. An optical head 60 has a semiconductor laser element 61 as the light beam source. The light beam radiated from the semiconductor laser element 61 is converged on a single point of the signal recording surface of the magneto-optical disc MO by an objective lens 62. The objective lens 62 may be a single lens having aspherical surfaces on both sides. The light beam irradiated by the semiconductor laser element 61 is then divided into at least three diffraction light beam i.e., a zeroth order diffraction light beam and a positive and a negative first order diffraction light bemas, by a diffraction grating 63 from the light beam emitted from the semiconductor laser element 61. For simplification, the three divided light beams are shown as a single light beam in FIG. 2. A beam splitter 64 is provided to separate light beam emitted from semiconductor laser element 61 and the reflected light beam both reflected from the signal recording surface of the magneto-optical disc MO. The divergent light beam is transmitted through the beam splitter 64 is converted into parallel light beams by a collimator lens 65. A Wollaston prism 66 is established on the beam splitter 64, on the end plane for emitting the reflected light beam. The Wollaston prism 66, including a pair of optical prisms 66a, 66b which are each made of lithium niobate (LiNbO 3 ), can be provided to the beam splitter 64 by adhesion, etc. The difference of indices of refraction Δn of this Wollaston prism is found to be: Δn ≡ n e - n o = 0.08, where, n e , the index of refraction of an extraordinary light beam, is 2.179 and n o , the same of an-ordinary light beam, is 2.259. This difference of indices of refraction is a digit larger as compared with that of an artificial quartz (SiO 2 ). Accordingly, the angle of the apex at the joint between the adhered prisms 66a and 66b of the Wollaston prism 66 can be lowered considerably to a value as small as, for example, 9 degrees. This can be seen in FIG. 3 (A). Accordingly, not only the thickness of the Wollaston prism 66 along the optical axis can be reduced, but also the thickness along the optical axis of the reflected light beam, which is illustrated in FIG. 2, can be reduced to, for example, 0.85 mm. Thus, the beam separation angle  3 of the Wollaston prism as shown in FIG. 3 (A) can be found to be, for example, 25.3 mrad. The Wollaston prism 66 according to the present embodiment includes two prisms 66a and 66b adhered to each other. The prism 66a on the side of light beam incidence for a reflected light beam and the like is rotated as such that the crystal optical axis thereof may make an angle of 45° with respect to the P-polarization plane of the incident light beam. The prism 66b provided on the output side for a light beam has, as shown in FIG. 3 (C), its crystal optical axis being rotated at an angle of 90° with respect to the P-polarization plane. The prisms 66a and 66b are adhered in such a manner that the respective crystal optical axes thereof may make the angle of 45° and 90°, respectively, with respect to the P-polarization plane of the incident light beam. Thus, as shown in FIG. 3 (A), the incident light beam is generated by the Wollaston prism 66 into three light beams each containing the polarization components, namely, P, S, and (P+S). A multi-purpose lens 67 is provided to impart stigmatic aberration to the output light beam of the Wollaston prism 66. The multi-purpose lens 67 also shortens the optical path of the reflected light beam in the same manner as in the previous example, by increasing the magnification. A photodetector 68 has a plurality of photoreceptor planes for receiving a plurality of the light beams generated by the Wollaston prism 66. A focusing error signal, a tracking error signal, and a read-out signal from the information signal recorded on the signal recording surface of the magneto-optical disc MO are generated according to the output signal from the photodetector 68. In the optical head 60 according to the present embodiment, the P-polarized light beam emitted from the semiconductor laser element 61 is introduced as a divergent light beam as it is into the diffraction grating 63 to generate at least three light beams, i.e., the zeroth order diffraction light beam and the positive and the negative first order diffraction light beams. The thus generated light beams are transmitted through the beam splitter 64 and converted into parallel light beams by the collimator lens 65. The light beam converted by the collimator lens 65 into parallel light beams is focused on the signal recording surface of a magneto-optical disc MO by the objective lens 62. In the signal recording surface, the zeroth diffraction light beam irradiates a beam spot on the track, accompanied by slightly offset beam spots attributed to the positive and the negative first order diffraction light beams located in the front and the back thereof. The light beams reflected by the signal recording surface, i.e., the reflected light beams undergo, due to Kerr effect, rotation of their polarization planes of the light beam along the direction of the magnetic domains of the signal recording surface. In this manner, the light beam includes an S-polarization component. The reflected light beam is introduced into the beam splitter 64 again via the objective lens 62 and the collimator lens 65. The optical path of the reflected is further deflected at the angle to 90° by the beam splitter 64, and introduced to the Wollaston prism 66. During its transmission through the Wollaston prism 66, the reflected light beam is generated into three light beams, i.e., P-polarization, S-polarization, and (P+S) polarization beams, attributed to the birefringence effect of the Wollaston prism 66. Since the three diffraction beams, i.e., the zeroth order diffraction beam and the positive and negative first order diffraction beams, are each further resolved into P-polarization, S-polarization, and (P+S) polarization beams attributed to the birefringence induced by the Wollaston prism 66, nine light beams in total are generated as a result from the Wollaston prism 66. The generated light beams are received by the photodetector 68 via a multipurpose lens 67. Among the nine light beams generated by the Wollaston prism 66, at least five of them, i.e., three zeroth order diffraction beams and two first order diffraction beams, are used in the practical signal processing. More specifically, the output signal from a plurality of photoreceptors which receive the light beam attributed to the center zeroth order diffraction is used to generate the focusing error signals; the output signal from the photoreceptor portion which receive two beams attributed to the zeroth order diffraction beam and located at both sides of the center zeroth order diffraction beam is used to generate the read-out signal from the information signal recorded on the magneto-optical disc MO; and the two light beams based on the first order diffraction is used for tracking error signals. According to the present invention, a thinner Wollaston prism can be employed by using a material having a relatively large refraction index, i.e., lithium niobate. Thus, the optical path between the Wollaston prism 66 and the photodetector 68 can be shortened. As a result, a compact optical head 60 can be realized. The beam splitter 64 may also function as an enhancement element in the same manner as in EP-A-0 620 550. In such a case, the beam splitter 64 should be constructed from a pair of optical prisms and a multilayered dielectric film. The multilayered dielectric film in this case is preferably designed as such that it may yield a transmittance of the P-polarization component (TP) of 45 %, a reflectivity of the P-polarization component (RP) of 45 %, a transmittance of the S-polarization component (TS) of several percentage, and a reflectivity of the S-polarization component (RS) of 90 %. While the invention has been described in detail and with reference to a specific embodiment thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein, the scope being determined by the appended claims.

Claims: An optical head apparatus for a magneto-optical recording medium, said apparatus comprising: a light beam source (61) for emitting a light beam; an objective lens (62) which focuses the light beam emitted from said light beam source (61) to the recording surface of a magneto-optical recording medium (MO); a separating optical system (63-67) which separates the beam into a light beam emitted from said light beam source (61) and a reflected light beam reflected from the recording surface of the magneto-optical recording medium (MO) via said objective lens (62); and a photodetector (68) for receiving said reflected light beam reflected from the recording surface, being separated by said separating optical system (63-67), wherein said separating optical system (60-67) comprises a Wollaston prism (66) made of lithium niobate placed in the upstream side of said photodetector (68) and in the optical path of said reflected light beam reflected from the recording surface, and wherein said Wollaston prism (66) is formed by adhering a pair of lithium niobate optical prisms (66a, 66b) to each other, in such a manner that the crystal optical axis of the prism on the side for light beam incidence of said reflected light beam from the recording surface is tilted at an angle of 45° with respect to the polarization plane of said reflected light beam from the recording surface, and that the crystal optical axis of the prism on the side for emitting said reflected light beam from the recording surface is tilted at an angle of 90° with respect to the polarization plane of said reflected light beam from the recording surface. An optical head apparatus according to claim 1, wherein said separating optical system (63-67) comprises a beam splitter (64) for resolving the beam into a light beam emitted from said light beam source (61) and a reflected light beam reflected from the recording surface of the magneto-optical recording medium (MO) via said objective lens (62), and said Wollaston prism (66) is provided on the output plane of said beam splitter (64) for irradiating said reflected light beam from the recording surface.

Inventors: ANDO NOBUHIKO, HINENO SATOSHI, ANDO, NOBUHIKO, HINENO, SATOSHI
Applicants: SONY CORP, SONY CORPORATION
Application year: 1994
Main classification: G11B11
IPC codes: G11B7, G11B11